ISSN (Print) : 0974-6846
Indian Journal of Science and Technology, Vol 8(23), DOI: 10.17485/ijst/2015/v8i23/70769, September 2015 ISSN (Online) : 0974-5645
Enhancement of Power Quality through
Shunt Compensators and Main Grid
Interfaced with DGs
K. Satish Reddy*, A. Appa Rao and G. Keshava Rao
Department of Electrical and Electronic Engineering, KL University, Vaddeswaram,
Guntur - 522502, Andhra Pradesh, India; sathi250@gmail.com
Abstract
Injection of power generated by the wind turbine system into an electric grid mainly effects the power quality. The
performance of this wind turbine and its power quality is determined on the basis of its measurement of power ratings
as per Institute of Electrical and Electronics Engineers (IEEE) standards. The influence of the wind turbine in the grid
system concerning the power quality measurements are the active power, reactive power, variation of voltage, flicker,
harmonics, and electrical behavior of switching operation. To mitigate the power quality problems this paper proposes
the shunt compensator techniques. Here, the proposed system is verified experimentally using both Static Synchronous
Compensator (STATCOM) and Thyristor Switched Capacitor (TSC) compensators. This control schemes for grid connected
wind energy system is simulated using Matlab/Simulink.
Keywords: Grid Interconnection, Power Quality, Static Compensator, Thyristor Switched Capacitor, Wind Energy System
1. Introduction
With expansion in the interest for Electricity because
of expansion in populace and industrialization, the
Generation of power was truly a test now a day. In the
event that we need to expand the power produced in the
customary path1,2 i.e., by method for non-renewable vitality sources like coal, diesel, normal gasses and comparative
fossil energizes, the contamination builds which debases
the Environment and human way of life. The reasons for
power quality issues are by and large mind boggling and
hard to identify when we coordinate a wind turbine to
the network. Therefore, the power electronic based forced
commutated converters are preferred in distribution systems for maintainence of system stability, reliability and
quality of power as the point of common coupling.
A shunt device is a compensating device i.e. which
is connected between the grid connected point called
*Author for correspondence
as Power Control Center (PCC) and the ground3. Shunt
device either can absorb or generate the reactive power
for controlling the magnitude of voltage at point of common coupling.
The reactive power compensation is also one of the
application of shunt converter devices4. Figure 1 shows
the basic diagram for the shunt connected inverter based
grid connected system.
2. Grid Interconnection of Wind
Energy System
Recently, grid connected wind system have been
spreading in residential areas and in industrial areas.
So we have to find a suitable Maximum Power Point
Technique (MPPT) technique that gives a better power
output. For a grid connected system, there are certain
factors that have been considered such that Direct
Enhancement of Power Quality through Shunt Compensators and Main Grid Interfaced with DGs
Figure 1. Diagram for Proposed System.
Current (DC)-Alternating Current (AC) conversion with
highest output power quality with the proper design
of filters. Grid interface inverters transfer the energy
from the wind energy generation system to the grid by
maintaining constancy of DC link voltage. For a grid
connected system the utility network mainly demands
for better power quality and power output. In the case of
voltage fluctuations, control of grid parameters is rather
difficult5. So for a wind system that is connected to a grid,
the first stage is the boosting stage and the second stage
is DC-AC converter6. An output filter is usually employed
which reduces the ripple components due to switching.
The problem associated with the grid connected system
is that the DC link voltage that oscillates between the
two levels depends on the operating climatic conditions
(ambient temperature & irradiance) in which inverter
that acts as a power controller between the DC link and
the utility isolated. DC link is generally used to isolate the
grid from the inverter side so that we can control both
wind system and grid separately. All the available power
that can be extracted from the wind system is transferred
to the grid6.
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3. Wind Energy System
The generation of electrical power is obtained mainly in
two ways. One is conventional source and other is nonconventional energy sources. The generation of electricity
using non-renewable resources such as coal, natural gas,
oil and so on, shows great impact on the environment
by production of green house gases. Hence, by considering all these conditions the generation of electricity is
obtained from the renewable energy sources.
Basically, out of all renewable energy sources the wind
turbine plays an important role for generating electricity.
And from economical point of view the wind turbine has
low maintainece cost and it needs no fuel, so, it is pollution free. In the world scenario, 50-60 percent7 of energy
is generated from wind turbine as compared with all other
renewable energy sources.
Figure 2 shows the basic configuration diagram of the
wind energy system.
The wind turbine converters wind energy to electrical energy and the generator mechanical shaft power is
obtained by the following expression:
Indian Journal of Science and Technology
K. Satish Reddy, A. Appa Rao and G. Keshava Rao
Figure 2. Basic schematic diagram of wind turbine.
Pm =0.5ρAv3Cp
Where (kg/m) is the air density and A (m) is the
area swept out by turbine blade, Vwind is the wind speed
in mtr/s. It is not possible to extract all kinetic energy of
wind, thus it extract a fraction of power in wind, there Cp
is the power coefficient, depends on type and operating
condition of wind turbine.
And the coefficient of power also plays a key role for
wind system and the basic minimum value of power coefficient is 0.58. The power coefficient is obtained by the
ratio of tip speed ratio to pitch angle. The pitch angle is the
angle at which the blades of turbine are arranged based on
their longitude axis and changes in wind direction. The
tip speed ration is defined as ratio of linear speed of the
rotor to the wind speed.
Figure 3 shows a typical waveform for coefficient of
power with respect to the Tip Speed Ratio (TSR). The
maximum achievable range of TSR is from 0.4 to 0.5 for
turbine with high speed and from 0.2 to 0.4 for turbine
with low speed9.
4. DFIG on Wind Generator
The doubly-fed induction has special advantages as
energy converter a wind generator for the following reasons; these make DFIG a preferred electro-mechanical
device and wind generators10.
• DFIG has the ability active or reactive powers independently through the rotor excitation currents.
Vol 8 (23) | September 2015 | www.indjst.org
Figure 3. Power coefficient Vs Tip Speed Ratio.
• The DFIG need not necessarily be excited from the
main grid. It can be magnetized from the circuit too.
• It is capable of generating reactive power that can be
delivered to the stator by the grid side converter.
• The size of the converter is not related to the converted
power; it is related to the selected speed range and
hence so the slip power.
• In case of weak-grids, where the voltage may fluctuate, the DFIG may be ordered to produce or absorb an
amount of reactive power from the grid with purpose
of voltage control.
The paper presented here addresses this problem.
When the weak systems, with large voltage fluctuations,
do not exchange adequate reactive power, the system is to
be supplemented with shunt compensating devices, like
Thyristor Switched Capacitor (TSC) or Static Synchronous
Compensator (STATCOM).
Indian Journal of Science and Technology
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Enhancement of Power Quality through Shunt Compensators and Main Grid Interfaced with DGs
4.1 STATCOM and its Control Technique
4.2 Thyristor Switched Capacitor
A STATCOM is a one of the compensated device which
is obtained from the Flexible Alternating Current
Transmission System (FACTS) family11 and is a combination of power electronic converter along with reactor.
Mostly, the converter is constructed by the use of fully
controlled devices such as Gate Turn-off Thyristor
(GTO), Insulated-Gate Bipolar Transistor (IGBT) or
Metal–Oxide–Semiconductor Field-Effect Transistor
(MOSFET). The main purpose of this STATCOM converter control technique is used to compensate the
deviations in power system for improving power quality. In this paper grid interfaced wind turbine based
STATCOM control scheme is proposed for improving the
reliability of electrical power12.
The major and most important component of the family of static var compensator14 is Thyristor Switched
Capacitor. It is an equipment which is used for reactive
power compensation in power systems. It is a combination of capacitor bank which is connected in series with
the anti-parallel thyristor valve and a small current limiting inductor.
The thyristor switched capacitor is commonly a three
phase system which is connected in either star or delta. As
compared with thyristor controlled reactor, the thyristor
switched capacitor produces less harmonics demanding
no extra filter circuits. Due to this reason the thyristor
switched capacitor is mostly used in static var compensator family and used for only reactive power15. The basic
schematic diagram for Thyristor Switched Capacitor
(TSC) is as shown in Figure 5.
• The DC voltage obtained for STATCOM is generated
from Solar Cells. The schematic diagram of Static
compensator is given in Figure 4.
The utilization of different types of electrical loads in
three phase system, produces an unbalances in current,
which causes the unreliable power13. Thereby for maintaining the electrical reliability the statcom controller
plays a key role. In this statcom control technique, the
reference voltage and dc link capacitor voltages are compared and the result obtained from this is converted to
two phase coordinators called as orthogonal vectors.
Figure 4. Basic block diagram for static compensator.
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5. Simulation Study
Case 1: Simulation result with use of TSC Converter
The simulation is done based on the Figure 1. The
simulation diagram for the proposed grid interfaced wind
energy system with Thyristor switched capacitor is shown
in Figure 6.
In Figure 7. the wave form (a) shows the output for
source current after compensation, (b) waveform for load
current, (c) waveform for the injected current by the TSC
converter and finally the waveform (d) shows the result
Figure 5. Thyristor Switched Capacitor.
Indian Journal of Science and Technology
K. Satish Reddy, A. Appa Rao and G. Keshava Rao
Figure 6. Simulation Diagram of proposed system with TSC.
Figure 7. Results for Wind Energy System with TSC.
Vol 8 (23) | September 2015 | www.indjst.org
Indian Journal of Science and Technology
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Enhancement of Power Quality through Shunt Compensators and Main Grid Interfaced with DGs
for current from wind turbine. And Figure 8. shows its
total harmonic distortion waveform.
Case 2: Simulation Diagram and results with
STATCOM
The simulation diagram for the proposed grid interfaced wind energy system with STATCOM controller is
shown in Figure 9.
Figure 8. FFT analysis for Source current.
In Figure 10 the wave form (a) shows the output for
source current after compensation, (b) waveform for
load current, (c) waveform for the injected current by the
Statcom converter and finally the waveform (d) shows
the result for current from wind turbine. Figure 11 shows
the total harmonic distortion of proposed system with
Statcom controller.
From these results, examined the performance of
TSC and STATCOM in electric power systems. Based
on the analytical and simulation studies, the impact
of TSC and STATCOM on the studied power system
is presented. It was shown that both devices significantly improve the transient voltage behavior of power
systems. And finally all these factors will increase the
reliability of wind power plants. So it seems inevitable to use TSC or STATCOM in power plants. Our
studies show that STATCOM have faster response in
compensation and correction of voltage profile than
TSC. But since our purpose is mainly to prevent system
instability in power systems so with regardless of the
cost and higher casualties of STATCOM we can prefer
STATCOM over TSC.
Figure 9. Over All Circuit Diagram in Simulation with Sub Systems.
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Vol 8 (23) | September 2015 | www.indjst.org
Indian Journal of Science and Technology
K. Satish Reddy, A. Appa Rao and G. Keshava Rao
Figure 10. Simulation result for current.
systems. The shunt devices proposed here, while reducing the distortions in currents, improved the power factor
thus reducing the reducing the reactive power demand
from the wind generator and the load at point of common coupling. Thus, the integration of FACTS devices
maintains the desired power quality requirements. The
operation of STATCOM and the TSC and their control
strategies are simulated in MATLAB/SIMULINK. The
use of STATCOM, as a shunt controller, gives better
results than by TSC.
7. References
Figure 11. FFT analysis for Source current.
6. Conclusion
The paper presents a novel concept of integration of
STATCOM/TSC with grid inter faced wind energy system
for power quality improvement. The paper also presents
effects of power quality on consumer and power utility
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